The Chamber is the first conservative group to wade into the Maine Senate race, which has been considered a safe bet for the popular King. With advertising inexpensive in Maine, officials familiar with the Chamber’s campaign suggested that the new ad wars could serve as an opening volley to see if King’s popularity can be dented and, if so, may soon be followed with campaign ads from other conservative groups.

“While King was governor, state spending skyrocketed to $2.6 billion; the king of mismanagement, when King left office, he left Maine with a $1 billion shortfall,” the narrator says in the Chamber’s ad, which began airing Thursday.

King’s campaign on Thursday called the ad “ill-informed” and “a case of Washington politicos trying to tell people in Maine how to vote.”

The $1 billion shortfall figure the ad cites doesn’t refer to an actual budget shortfall because Maine’s constitution requires that the state budget be balanced. However, had the Legislature that took over as King left office in 2003 funded all services at the same level, the state would have spent about $1 billion more than it was projected to take in in revenue.

The $2.6 billion state spending figure reflects the size of the state’s general fund in fiscal year 2001, when spending from the fund hit $2.57 billion, according to figures from the state’s Office of Fiscal and Program Review. During fiscal year 1997, the general fund’s size was $1.77 billion.

Grant Pennoyer, the Office of Fiscal and Program Review’s director, said the general fund grew by about $850 million during King’s administration, from 1995 to 2003. About half of that growth can be attributed to growth in state subsidies for school districts and the cost of funding Medicaid services. Growth in higher education funding and state government personnel costs — including salaries, health insurance and retirement benefits — also contributed.

The state general fund had to pick up a greater portion of Medicaid costs, Pennoyer said, after the state ended its assessment on hospitals in 1998. And some of the spending growth during the King administration can be attributed to the state stopping a practice in which it deferred school subsidy and other payments from one budget year to the next.

Pennoyer said that was a common “budget gimmick” that began in the early 1990s as a way to balance the budget from year to year. Ending that practice accounted for $39 million in school subsidy growth in fiscal year 1998, he said.

“These negative ads are business as usual in Washington, and it is exactly what needs to change,” King campaign spokeswoman Crystal Canney said in a statement. “The U.S. Chamber is the largest lobbying organization in America, and they refuse to disclose their donors. It is nameless, faceless outside money trying to influence Maine voters.”

The Maine Small Business Coalition, a liberal advocacy group, also condemned the U.S. Chamber ad.

“The U.S. Chamber does not speak for Maine businesses, and they don’t care about our economy,” coalition director Kevin Simowitz said in a statement. “The Chamber is an interest group for large, out-of-state and multinational corporations, especially health insurance companies.”

Summers’ campaign welcomed the U.S. Chamber ads. “This strong stand by the U.S. Chamber of Commerce makes it clear that this Senate race is on the map,” Summers campaign manager Lance Dutson wrote in an email to supporters.

U.S. Chamber officials declined to address the total cost of the ad campaign, but it is expected to run for nearly 10 days.

The Maine State Chamber of Commerce is a member of the U.S. Chamber of Commerce, but is not affiliated with the U.S. Chamber’s political arm that is sponsoring the King attack ad, said Maine Chamber President Dana Connors.

“We do not endorse candidates,” he said. “We stay out of that.”

Maine’s Senate race is one of 11 in which the Chamber has launched TV ad campaigns through its political arm, which can run its operations without disclosing the donors financing them. Maine, however, marks the riskiest bet yet because the other 10 races are considered the marquee match-ups that will decide which party controls the Senate next year. After Republican Sen. Olympia Snowe made the surprise announcement that she would not seek re-election, Republicans and Democrats scrambled to find candidates for what had previously been considered an easy race for the popular Snowe.

Summers, Maine’s secretary of state, has lost three previous bids for a seat in the House. He won the GOP primary last month, when Democrats nominated state Sen. Cynthia Dill of Cape Elizabeth.

King’s entrance into the race prompted prominent Democrats to back away from running, leading strategists in both parties to believe King’s unspoken plan is to support the Democrats. In Washington, the Democratic Senatorial Campaign Committee has done nothing to support Dill’s nomination, and the independent Rothenberg Political Report recently graded the race as shifting “toward Democrats — sort of” as it declared King the clear favorite.

King has led his two rivals in all recent polling. Most recently, he attracted the support of 55 percent of those surveyed in a poll conducted by the Portland firm Critical Insights between June 20 and 25. Summers had 27 percent support in that poll, and Dill had 7 percent.